Monstrous regiment

Polly Perks had to become a boy in a hurry. Cutting off her hair and wearing trousers was easy. Learning to fart and belch in public and walk like an ape took more time. And now she's enlisted in the army, and is searching for her lost brother. But there's a war on. And Polly and her fellow recruits are suddenly in the thick of it, without any training, and the enemy is hunting them down. This is the 28th Discworld novel.